1. Field of the Invention
The present invention relates to surveillance systems, and, more particularly, to surveillance systems for detecting an intrusion into a monitored area of space.
2. Description of the Related Art
Surveillance systems including motion detectors are known for detecting intrusions of a moving object, such as a human, into a monitored zone of space. The motion detectors typically include infrared detectors that sense the presence of a source of infrared radiation, e.g., a warm body, anywhere along the line of sight of the infrared sensors. FIG. 1a illustrates a typical motion detector 30 mounted on a wall 32 or a ceiling 34. Motion detector 30 monitors a zone 36 within a room by imaging multiple areas of the room onto an infrared sensor 38 shown in the enlarged view of FIG. 1b. The output of detector 30 is then amplified and processed for alarm output from the motion detector. Thus, the monitored zone 36 generally includes at least a majority of the space between motion detector 30 and some barrier, such as a wall 40 or a floor 42, that blocks infrared radiation.
The monitored space or zone is typically divided into a plurality of subzones, such as the illustrated subzones 44a-i, and the detector may detect movement from one of the subzones to another. Only the subzones extending through a central portion of the room are shown in FIGS. 1a and 1b for clarity of illustration. A single fresnel lens array 46 may be used to focus subzones 44 onto sensor 38. Although most of the space in the room may be monitored by this arrangement, a zone 48 may remain unprotected due to limits on the size of fresnel lens array 46. Thus, an intruder may be able to pass through the room undetected by walking through only unprotected zone 48.
It is possible to add another fresnel lens 50 (FIG. 2b) for looking downward from the sensor. However, as illustrated in FIG. 2a, unprotected zones 52 still remain which allow a skilled intruder to pass through the room undetected.
What is needed in the art is an inexpensive intrusion detection system that can monitor the length of a room, including the space below the detector, to prevent an intruder from passing through the room undetected.